If the world is from an older snapshot then it's probably just an old bugged out strider, but I haven't tested to see if the bug is still around. It was an error when striders got unloaded when they got reloaded they became "floaty".
Here is a full report of why this bug occurs.
Striders when they stand on lava get the NBT tag NoGravity:1b when they step off it gets removed.
How ever if a strider gets unload while they have the NBT NoGravity:1b, when they get reloaded they will become floaty.
Unloading and Reloading such as Save and Quit, Leave the Nether, or have a chunk unloaded by walking out of range, ect.
You can band-aid this by having a repeating command block run the following command:
/execute as @e[type=strider,nbt={NoGravity:1b}] run data merge entity @s {NoGravity:0b}
Hope this helps! 🙂
Stiders have NoGravity:1b but for some reason they are unaffected by it till they are unloaded and reloaded such as leaving the Nether or Save and Quit.
Still in 20w15a
Yes but if it is fixed, it won't fix old broken striders only new spawned ones should be fixed.